VPB 112: General Veterinary Biochemistry (1+1)

TITRATION CURVE – THEORETICAL CONSIDERATIONS

• At the beginning of the titration, before NaOH is added, very small quantity of acetic acid is ionized.
• As NaOH is gradually introduced, the added OH^- ion combines with the free H⁺ ion in the solution to form water,

CH₃COOH → CH₃COO^- + H⁺

K_a= [([H⁺] [CH₃COO^-] ) / [CH₃COOH] ]= 1.74 x 10^-5 M

• As free H⁺ ions are removed, acetic acid dissociates further to make K_a as constant. As the titration proceeds by the addition NaOH more and more acetic acid ionizes, forming acetate.
• At the mid point of the titration at which exactly 0.5 equivalent of NaOH has been added, one half of the original CH₃COOH has undergone dissociation so that the concentration of the proton donor (CH₃COOH) now equals that of the proton acceptor (CH₃COO^-). The pH of the equimolar solution of acetic acid and acetate is exactly equal to the pKa of acetic acid (4.76)
• As the titration is continued by the further addition of NaOH, the remaining undissociated acetic acid is gradually converted into acetate ions.
• The end point of the titration occurs at about 7, in which all the acetic acid has lost its proton to OH^- ions, to form water and acetate.
• The titration curve is used not only to find out the pKa of a weak acid but also shows graphically that a weak acid and its conjugate base can act as a buffer.